Project Summary/Abstract A successful vaccine against tuberculosis (TB) must protect the most vulnerable populations, including those with HIV infection. TB accounts for a disproportionate number of deaths among HIV-infected individuals. Even during suppressive antiretroviral therapy (ART), HIV-infected individuals remain at higher risk of progressive TB compared to cohorts without HIV infection. Little is known about how pre-existing HIV infection influences the outcome of subsequent M. tuberculosis (Mtb) infection, which will be a formidable obstacle in vaccine development. In response to PAR-16-254, ?Mechanisms of mycobacterial-induced immunity in HIV-infected and uninfected individuals to inform innovative tuberculosis vaccine design?, this proposal will focus on characterizing the influence of suppressed or unsuppressed simian immunodeficiency (SIV) infection on Mtb infection in a macaque model. Our overarching hypothesis is that pre-existing SIV infection will affect the outcome of subsequent Mtb infection. The risk of progressive TB is greatest when SIV replication is unsuppressed, resulting in impaired immune responses and, thus, high bacteria burden and dissemination. We propose to perform serial in vivo imaging and molecular techniques to track individual Mtb bacilli within individual granulomas and other tissues and correlating them with SIV replication and evolution and immune responses. In Aim 1, we will assess both how pre-existing unsuppressed or suppressed SIV infection alters the pathogenesis of Mtb infection and how subsequent Mtb infection alters SIV replication and diversity during co- infection. In Aim 2, we will characterize the innate and adaptive immune responses during the course of both infections as well as SIV induced immune activation factors to identify the immunologic deficits present in suppressed or unsuppressed SIV and Mtb co-infection. Lastly, in Aim 3, we will correlate the dynamics of SIV replication, Mtb growth and dissemination, and immunity within granulomas and other tissues over time and examine the effects of Mtb infection (bacterial growth, killing and dissemination) and SIV viral replication and evolution in the context of immune responses in granulomas and other infection sites. By comparing these outcomes between suppressed and unsuppressed SIV infection and SIV uninfected groups, we will have a better fundamental understanding of the immunologic and pathogenic features of risk during HIV-Mtb co- infection. Results of these studies should directly translate to humans with either suppressed or unsuppressed HIV infection who live in TB endemic areas. This proposal will also provide important insights to the immunologic deficits in pre-existing HIV infection that are critical to controlling Mtb infection. Such findings will be essential in the strategic development and design of future vaccine vaccines that will protect HIV-infected individuals against TB.